This disclosure generally relates to exploration and production of hydrocarbons involving investigations of regions of an earth formation penetrated by a borehole. More specifically, the disclosure relates to the accurate measurement and imaging of the resistivity of an earth formation using a logging tool in a borehole.
Generally, there are two categories of electrical logging apparatuses. In one category, one or more measurement electrodes, such as current sources or sinks, are used in conjunction with a return electrode. A measurement current flows in a circuit that connects a current source to the measurement electrode(s), through the earth formation to the return electrode, and back to the current source in the tool. In another category, electrical logging apparatuses include inductive measuring tools and an antenna, which are configured to induce a current flow within the earth formation.
Electrical logging devices may be operable in one or more modes. In one mode, a constant current is maintained at a measuring electrode while a voltage is measured; in another mode, the voltage of the measuring electrode is held constant and the current is measured. Ideally, if current is varied to maintain a constant voltage, the resultant current is inversely proportional to the resistivity of the earth formation being investigated. Conversely, if current is maintained constant through the circuit, ideally speaking the voltage of the measurement electrode is proportional to the resistivity of the earth formation being investigated. Ohm's law teaches that if both current and voltage vary, the resistivity of the earth formation is proportional to the ratio of the voltage to the current.
In real world applications, however, the aforementioned physical relationships are more complicated. Those of ordinary skill in the art will appreciate that there are usually other variables to consider, such as, for example, electrical characteristics of the instrumentation itself and electrical characteristics of the environment under consideration.
It is known that oil-based drilling fluids (e.g., muds) may be used when drilling through formations and an increasing number of present day exploration prospects use such techniques. The use of oil-based muds, which have a low conductivity relative to water-based muds, implicates a potential drawback in the use of contact electrodes to inject electrical currents into a wellbore. Any standoff or gap between an electrode and a wellbore creates a standoff impedance; and as electrode standoff increases, the corresponding impedance can begin at some point to dominate resistivity measurement taken through the electrode. Increasing rugosity which tends to increase average electrode standoff, thus becomes especially problematic.